In an oscillation damper, particularly in an oscillation damper of a motor car, a piston unit separates two working chambers from each other. A damping valve unit extends across said piston unit. The piston unit is axially movable together with a piston rod member with respect to a cylinder member accommodating the piston unit within a cavitiy. The damping characteristic depends not only on the construction of the damping valve unit but also on the volume of fluid which is urged through the damping valve unit per time unit. The volume of damping fluid to be urged through the damping valve unit per time unit can be varied by a by-pass channel arranged substantially parallel with respect to the damping valve unit. The by-pass channel contains a by-pass valve unit. The by-pass valve unit can be switched between an open position and a closed position. In the closed position a maximum flow of damping fluid occurs through the damping valve unit and the damping action of the oscillation damper is at a maximum damping force. In the open condition of the by-pass valve unit a considerable part of the damping fluid can flow through the by-pass channel. So the damping action is at a minimum damping force. The switching between the closed condition and the open condition of the by-pass valve unit requires a finite transition time or switch time. During this switch time the cross-sectional area of the by-pass valve unit is varied.